Real-time X-ray monitoring

ABSTRACT

A medical imaging system has a radiation source, a radiation sensor, a data-collection unit, and an imaging system. The radiation source has an opening to direct a collimated radiation beam in a direction towards a patient. The radiation sensor is disposed proximate the opening and within the collimated radiation beam to measure a fluence of the collimated radiation beam. The data-collection unit is disposed to collect radiation from the collimated beam after interaction with the patient. The imaging system is in communication with the data-collection unit and configured to generate an image of a portion of the patient from the collected radiation.

BACKGROUND OF THE INVENTION

This application relates to diagnostic radiation systems such as x-raysystems. More specifically, this application relates to methods andapparatus for monitoring the operation of diagnostic radiation systems.

Ever since Wilhelm Rontgen discovered x rays and successfully imaged hiswife's hand to show the structure of her bones, radiation has been usedas a medical diagnostic tool. While two-dimensional radiographs wereused for decades, such images suffered from the superposition of imagesof structures outside the specific region of interest and were generallyproduced images that were limited to particular image planes.

More recent advances have resulted in the development of tomographictechniques, particularly as embodied in computed-tomography (“CT”)imaging devices and in computed axial tomography (“CAT”) imagingdevices. Since their introduction in the 1970's, tomographic imagingdevices have become widely used for both diagnostic and preventivemedical applications. In addition to perform CT and CAT scans to confirmsuspected diagnoses of tumors, infarction, bone trauma, and the like,scanning using such devices is now almost routine for patients at highrisk for certain medical conditions such as colon cancer and heartdisease. Indeed, some institutions offer full-body scans to the generalpublic as part of a generalized effort for early detection of disease.

While such efforts have had a significant impact in allowing physiciansto detect disease early and to confirm diagnoses without invasivetechniques, they are not without a number of concerns. One particularconcern results from the fact that x rays are a form of ionizingradiation that have their own impact on the body being measured. Sincethe early 1980's, the per capita dose of radiation from medical imaginghas increased by a factor of almost six. Some estimates suggest that thecurrent level of usage of CT scans will result in an increase in cancermortality rate of 1.5% to 2% from cancers caused by the scans. While thebenefit of reducing cancer mortality from early detection of cancerssignificantly exceeds this rate, it remains a concern.

Monitoring the actual dose delivered to patient is complicated by anumber of factors. The dose depends on multiple known factors thatinclude the volume and type of tissue scanned, the build of the patientscanned, the number and type of scan sequences, and the quality ofimages to be produced. There is, moreover, a lack of uniformity amongmachines used to perform the scans, varying not only among manufacturersbut also being sufficiently complex devices to have individualvariations in uniformity. The dose received by a patient depends on howthe machines are used and how different settings for a particularimaging session are configured.

In addition to these patient concerns, there are concerns about themachines themselves. The x-ray tube, for example, tends to degrade overtime as the machine is used. To obtain a similar image quality, amachine tends to need to be operated at higher current (mA) as theefficiency of the tube decreases. It is desirable to be able to predictwhen tube operational quality is likely to become so low thatreplacement is needed.

There are, thus, a number of deficiencies in the art that it isdesirable to address.

SUMMARY

Embodiments of the invention provide a medical imaging system that has aradiation source, a radiation sensor, a data-collection unit, and animaging system. The radiation source has an opening to direct acollimated radiation beam in a direction towards a patient. Theradiation sensor is disposed proximate the opening and within thecollimated radiation beam to measure a fluence of the collimatedradiation beam. The data-collection unit is disposed to collectradiation from the collimated beam after interaction with the patient.The imaging system is in communication with the data-collection unit andconfigured to generate an image of a portion of the patient from thecollected radiation.

The radiation sensor may comprise a scintillating fiber that emits lightin response to absorption of a photon of radiation by the scintillatingfiber. A photodetector is coupled with the scintillating fiber to detectemission of light by the scintillating fiber. In some instances, thescintillating fiber may comprise a plurality of scintillating fibersarranged substantially parallel to each other and the photodetector maycomprise a plurality of photodetectors with each of the plurality ofphotodetectors being coupled with one of the plurality of scintillatingfibers.

In some embodiments, the radiation sensor has two such arrangements indifferent orientations. Specifically, the radiation sensor comprises afirst radiation sensor and a second radiation sensor. The firstradiation sensor has a plurality of scintillating fibers arrangedsubstantially parallel to each other and to a first direction, with eachof the first plurality of scintillating fibers emitting light inresponse to absorption of a photon. Each of a first plurality ofphotodetectors is coupled with one of the first plurality ofscintillating fibers to detect emission of light by the one of the firstplurality of scintillating fibers. The second radiation sensor has asecond plurality of scintillating fibers arranged substantially parallelto each other and to a second direction, with each of the secondplurality of scintillating fibers also emitting light in response toabsorption of a photon. Each of a second plurality of photodetectors iscoupled with one of the second plurality of scintillating fibers todetect emission of light by the one of the second plurality ofscintillating fibers. The first and second directions are nonparallel.In a particular embodiment, the first and second directions may besubstantially orthogonal.

The medical imaging system may additionally comprise a monitoring systemin communication with the radiation sensor. The monitoring system hasinstructions to determine an estimate of an effective radiation dosedelivered to the patient during an imaging procedure with the medicalimaging system from the measured fluence. The radiation sensor maymeasure a spatial distribution of the fluence and/or it may measure aspectral distribution of the fluence. The medical imaging system mayalso additionally comprise a mechanism to effect relative translationaland/or rotational motion between the radiation source and the patient.

To determine the estimate of the effective radiation dose delivered tothe patient, a number of quantities may be obtained: a peak voltageapplied to the radiation source to generate the collimated radiationbeam, a measure of a geometry of the medical imaging system, a measureof a size of the patient, and a measure of relative motion of thepatient with respect to the medical imaging system. In one embodiment,the medical imaging system further comprises a host system incommunication with the imaging system and with the radiation source,with each of these quantities being obtained from the host system.

In other embodiments, each of the quantities is instead obtained from anappropriate sensor also comprised by the medical imaging system. Forexample, the peak voltage applied to the radiation source may beobtained from the measured fluence of the collimated radiation beam.This may include, for example, using spectral information such as thehalf-value-layer-aluminum. The medical imaging system may furthercomprise a geometry sensor, with the measure of the geometry of themedical imaging system being obtained from the geometry sensor; examplesof suitable geometry sensors include an ultrasound sensor, a lasermicrometer, and a visual camera. The medical imaging system may furthercomprise a patient-size sensor, with the measure of the patient sizebeing obtained from the patient-size sensor; examples of patient-sizesensors also include an ultrasound sensor, a laser micrometer, and avisual camera. The medical imaging system may further comprise a motionsensor, with the measure of relative motion of the patient with respectto the medical imaging system being obtained from the motion sensor; themotion sensor might comprise a mechanical sensor, an electromagneticsensor, or an acoustic sensor. The medical imaging system may furthercomprise a motion sensor for determining gantry rotation.

The monitoring system may form part of a wider, centrally organizedsystem. Specifically, the monitoring system may be in furthercommunication with a central system that is in communication with asecond monitoring system remote from the monitoring system. In suchinstances, the monitoring system may have instructions to record theestimate of the effective radiation dose delivered to the patient duringthe imaging procedure at a data store coupled with the central system.

The monitoring system may also may have functionality in addition todetermine dose estimates. For example, the monitoring system mayidentify the measured fluence of the collimated radiation beam as beingoutside an acceptable range, initiating an alarm in response to such anidentification. In other instances, the monitoring system may perform acomparison of the measured fluence of the collimated radiation beam witha record of prior measurements of fluence produced by the radiationsource and thereby estimate a time to failure of the radiation sourcefrom the comparison.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings, wherein like reference labels are usedthrough the several drawings to refer to similar components. In someinstances, reference labels are followed with a hyphenated sublabel;reference to only the primary portion of the label is intended to refercollectively to all reference labels that have the same primary labelbut different sublabels.

FIG. 1 provides a schematic overview of a CT scanning system as may beused in embodiments of the invention;

FIG. 2 illustrates a system in which the CT scanning system illustratedin FIG. 1 may be integrated;

FIG. 3 is a schematic illustration of a structure that may be used for amonitoring system used as part of the CT scanning system of FIG. 1;

FIG. 4 shows a structure that may be used for an x-ray sensor used withthe CT scanning system of FIG. 1;

FIG. 5 illustrates measurement of an x-ray beam profile with prior-artdetection methods;

FIG. 6 is a flow diagram summarizing methods of the invention in someembodiments;

FIG. 7 provides an illustration of a typical spectral distribution offluence for an x-ray tube; and

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of the invention are directed generally to methods andapparatus for monitoring the operation of diagnostic radiation systems.While much of the discussion herein focuses on CT imaging devices, it isto be understood that this is by way of illustration only. Moregenerally, the principles applied with the invention may be implementedin a variety of different types of diagnostic systems that use ionizingsources of radiation.

One example of a CT system 100 adapted in accordance with embodiments ofthe invention is illustrated in FIG. 1. In this drawing, the imagingsystem is shown generally in the middle of the drawing and is adaptedfor irradiation of a patient 104 in a variety of different modes.Radiation is supplied by a radiation source 112, which may be an x-raytube of the type well-known in the art. Radiation emitted by the source112 is collimated by a collimator 116 to direct it towards the patient104. A filter 120 may optionally be included for spatially varyingattenuation of the generated radiation beam. For example, because thehuman body is generally thicker in the middle and narrower around theedges, a “bowtie” filter may be used to attenuate the radiation beam atits edges while allowing stronger fluence to propagate near the centerof the beam.

There may be multiple degrees of freedom of motion, both rotational andtranslational. Typically, for example the patient 104 is positioned on atable 108 that may move translationally while the beam is subject torotational motion with a gantry 118 to which the radiation source 112 iscoupled. This particular separation of rotational and translationalmotion is not a constraint of the invention, which may be implementedequally well in systems that may be deployed with other mechanisms forachieved the desired motion. When the table 108 is moved but the gantry118 is stationary, the beam effectively moves translationally throughthe patient 104, enabling a series of image slices to be derived. Whenthe gantry 118 is in motion but the table 108 is stationary, the beammoves rotationally, enabling multiple orientation images of structuresto be derived to produce an effective three-dimensional image. These canbe combined when both the table 108 and gantry 118 are in motion,producing an effective helical beam used in imaging the patient.Generally, both types of motion are at constant rates, although theinvention is not limited to such uses and may be adapted to specializedapplications as might be developed for varying rates of translationaland/or rotational motion.

The various components of the structure are controlled with modules thatinclude a voltage generator 124, a gantry driver 128, a table driver132, and an image generator 140 that is interfaced with a datacollection unit 136. The voltage generator 124 provides tube voltage andcurrent to the radiation source 112 according to instructions receivedfrom a host system 144 described in greater detail below. The tubecurrent and voltage are provided in accordance with a mode of operationof the CT system and may vary for different applications. Specificvalues of the tube current and voltage define the fluence intensityemitted by the radiation source 112. Typical values are 50-150 kV and100-500 mA, but embodiments of the invention may also use values outsideof these ranges.

The gantry driver 128 effects rotational motional of the gantry 118 andmay be configured to implement a number of different modalities. Forexample, the gantry driver 128 may be configured to rotate the gantry adefined forwards or backwards angle so that images may be derived withthe radiation source 112 in a specific position relative to the patient104. Alternatively, the gantry driver 128 may be configured to rotatethe gantry 118 continuously for a period of time at a predeterminedrate. Typical rates may be around 0.1-2.0 seconds/rotation, butembodiments of the invention may also use values outside of this range.

The table driver 132 similarly effects translational motion of the table108. The principal translational motions effected by the table driver132 are in a longitudinal direction, i.e. orthogonal to the page in thedrawing, and may provide both discrete motions and continuous motions.Specifically, discrete motions may be used so that the patient 104 ispositioned relative to the radiation source 112 for imaging of a definedportion of the body. Continuous motions may be used for imaging of agreater portion of the body by taking slice images as noted above. Inaddition, it is possible for the table driver 132 to be configured toeffect other translational motions of the table 108. For instance, thetable driver 132 may be configured to raise or lower the table 108,enabling the patient to be positioned at a desired distance from theradiation source 112. This may be particularly useful in accommodatingpatients of different sizes so that preferred imaging geometries may beachieved. In addition, the table driver 132 might also be configured fortransfer motion of the table 108 to further refine the desired imaginggeometry.

The data collection unit 136 may take different positions in differentembodiments, depending particularly on acceptable scattering angles forthe detected radiation being used for image generation. One position forthe data collection unit 136 is beneath the table 108. The datacollection unit 136 may take a variety of forms, one example of whichcomprises an array of radiation-detector elements matched to the spreadof a beam irradiated from the radiation source 112.

The image generator 140 is provided in communication with the datacollection unit 136 to apply processing methodologies to the collecteddata in generating an image. Such methodologies may include such knowntechniques as volume rendering, multiplanar reconstruction,minimum-intensity projection, computed-volume radiography, and the like.For three-dimensional images generated by the image generator 140,relevant supplementary information is generally associated. This mayinclude, for example, information identifying the patient 104. It mayalso include information defining a visual point of a three-dimensionalimage obtained from the relevant data and a line of sight determinedfrom the visual point, as well as information of an image-takingdirection corresponding to the line of sight, etc.

These various modules are provided in communication with a controlsystem, shown in the drawing as comprising a host system 144, an inputdevice 148, and a display control 152. The host system 144 has access toa storage device 164, which may form part of the control system or whichmay be separately accessible by the host system 144. One function of thehost system 144 is to control the various modules used in defining theimaging geometry, i.e. the gantry driver 128 and the table driver 132,the voltage generator 124 to define the operational characteristics ofthe imaging procedure, and the image generator 140 itself in definingthe type and quality of images to be generated from the procedure.

The host system 144 effects such control by running software that mayadditionally obtain input parameters from an operator in accordance withthe type of imaging to be performed so that the generated images arediagnostically relevant. Such parameters may be provided through theinput device 148, which may take a number of different forms indifferent embodiments. For example, the input device 148 may comprise atouch panel that displays input content with figures or charactersselected by the operator, or might comprise a keyboard or other type ofinterface that allows the receipt of setting values, instructions, andthe like from the operator. The display control 152 interacts with theimage generator 140 to cause a display of the images produced by theimage generator 140 to be shown on a display 156.

The information stored on the data store 164 may vary among differentembodiments. In some instances, the data store 164 is used exclusivelyfor the storage of information related to configuring the CT system foran imaging procedure, including software that is run by the host system144 and a record of parameters used to define certain imagingprocedures. Such information includes voltage and current specificationsfor the radiation source 112, geometry specifications that includewhether there is to be relative motion between the radiation source 112and the patient 104, the type of image to be generated and the like.

In addition to the various components described above, embodiments ofthe invention additionally include one or more sensors identifiedgenerally on the left of the drawing. Such sensors enable an independentdetermination parameters relevant to determining the radiation doseadministered to the patient 104 during an imaging procedure. While theremay be instances in which such parameters can be obtained from thecontrol system directly, the deployment of additional sensors asdescribed here enables such parameters to be determined without beingsupplied by the control system.

A radiation sensor 168 is provided at the output of the collimator 116and is configured to measure the fluence of the generated radiationbeam. In some instances, the fluence may be measured in a singledimension, particularly along the longitudinal direction in which thepatient may be moved by the table driver 132. Alternatively, the fluencemay be measured in multiple dimensions, particularly along thelongitudinal direction but also along the transverse direction, i.e.orthogonal to the longitudinal direction and parallel to the plane ofthe table 108. Fluence measurements are generally taken concurrentlywith operation of the system, thereby providing a real-time measurementresult of the radiation intensity exiting the collimator 116. If abowtie or other type of filter is incorporated as discussed above, theradiation sensor 168 may be provided at the output of such a filter.

This information may be combined with information derived from othersensors, which are included with some but not all embodiments of theinvention. For instance, a geometry sensor 172 may be used to measurethe physical separation of different components of the system,particularly in relation to a position of the patient 104. Apatient-size sensor 176 may similarly be deployed to determine physicalmeasurements of the patient 104 and a motion sensor 180 may be used todetermine rates of rotational or translational motion of the differentcomponents of the system. A rotational-velocity sensor 182 may determinerates of rotational motion of the gantry 118. Each of these additionalsensors may thus provide further information relevant in determining theactual radiation dose administered to the patient when combined withinformation from the radiation sensor regarding the fluence of theradiation beam.

Operation of the various sensors 166, 172, 176, 180, and 182 may becoordinated with a monitoring system 184. The dashed line in the drawingindicates that in some embodiments the monitoring system may be providedin communication with the host system 144, using a wired or wirelessconnection. When such communication is provided, the monitoring system184 may exchange information with the control system, such as by usingthe database 164 coupled with the host system to store relevantinformation and/or by obtaining information about the parameter settingsfor an imaging procedure. In cases where such information is available,the monitoring system 184 may use information derived from the sensorsas a form of verification and calibration of dose relationships. As willbe apparent from the discussion below, the sensor information is capableof providing more accurate dose determinations than the parameterinformation used by the host system 144 in configuring the CT system.

The monitoring system 184 may be one of a plurality of monitoringsystems that are used to monitor different CT systems. This isillustrated in FIG. 2, which shows a plurality of monitoring systems 184in communication with a central system 204 through a network 200. Thenetwork 200 may comprise a public network such as the Internet in someembodiments, or may comprise a private network. Because some of theinformation may be sensitive, particularly when patient information isincluded, it is preferable to use an encryption or other type ofsecurity system for communications between the monitoring systems 184and the central system 204 at least when the network 200 comprises apublic network.

With the networked arrangement illustrated in FIG. 2, a database 208coupled with the central system 204 may be used to integrate informationobtained from different monitoring systems 184. Such integration may beparticularly useful when monitoring systems 184 are being used tocollect information from CT systems produced by the same manufacturer,enabling statistical methods to be applied to the collected data inimproving dose, lifetime, and other determinations as described below.In addition, when dose information is associated with particular patientinformation, the centralized nature of the database 208 permits lifetimepatient information to be monitored, even when the patient may haveimaging procedures performed at different locations or facilities. Sucha capability allows more accurate information to be provided to patientsand physicians about lifetime exposure to medical-imaging radiation thatmay accordingly be a factor in evaluating the risks of futureprocedures.

A structure that may be used for each of the monitoring systems 184 isshown schematically in FIG. 3. This drawing broadly illustrates howindividual system elements may be implemented in a separated or moreintegrated manner. The monitoring system 184 is shown comprised ofhardware elements that are electrically coupled via bus 326. Thehardware elements include a processor 302, an input device 304, anoutput device 306, a storage device 308, a computer-readable storagemedia reader 310 a, a communications system 314, and a processingacceleration unit 316 such as a digital-signal processor orspecial-purpose processor. The computer-readable storage media reader310 a is further connected to a computer-readable storage medium 310 b,the combination comprehensively representing remote, local, fixed,and/or removable storage devices plus storage media for temporarilyand/or more permanently containing computer-readable information. Thecommunications system 314 may comprise a wired, wireless, modem, and/orother type of interfacing connection and permits data to be exchangedwith external devices.

The monitoring system 184 also comprises software elements, shown asbeing currently located within working memory 320, including anoperating system 324 and other code 322 that may be loaded into workingmemory on bootup or loaded separately. Such other code may comprisecomputer programs designed to implement methods of the invention. Itwill be apparent to those skilled in the art that substantial variationsmay be used in accordance with specific requirements. For example,customized hardware might also be used and/or particular elements mightbe implemented in hardware, software (including portable software, suchas applets), or both. Further, connection to other computing devicessuch as network input/output devices may be employed.

A structure that may be used for the radiation sensor 168 is illustratedin FIG. 4. This structure comprises a plurality of scintillating fibers404 coupled at each end with a photosensor 408. Each scintillating fibercomprises a tube that includes a core of scintillating material,possibly including additional dopants to promote scintillation. Thescintillating fiber may be made of plastic, and can be homogeneous ormade from a plurality of different plastics by having an inner core anda cladding sheath. Scintillating material responds to absorption ofradiation by emitting radiation, usually less energetic than what isabsorbed. This emitted radiation is detected by the photosensors 408 atthe ends of the scintillating fibers, enabling detection of the incidentfluence from the radiation source 112. The light intensity detected bythe photosensors 408 is a function of the energy and number of photonsabsorbed, with the number of absorbed photons itself being proportionalto the incident fluence and the length of the fiber 404 exposed to theradiation. The light thus generated is converted into an electricalsignal for further processing by the photosensors 408. The electricalsignal may be amplified with an amplifier 412 and digitized with adigitizer 416 for use by the monitoring system 184.

The use of scintillating fibers advantageously provides a radiationsensor 168 that can be placed directly in the beam path at thecollimator output, between the radiation source 112 and the patient 104,because it has very low and relatively uniform x-ray attenuation. Thiscontrasts with conventional detectors that use electronic components,circuit boards, and the like. Such structures are made with copper andother heavy metals that significantly attenuate x rays. By using ascintillating-fiber-based radiation sensor 168, such materials may bedisposed elsewhere—outside the x-ray beam. Keeping the electronics outof the x-ray beam also improves the long-term reliability of theradiation sensor 168 because continuous or frequent exposure tohigh-energy photons can damage and degrade electronic components.

In the illustration, a plurality of fibers 404 are included to provide ameasure of the spatial variation of the fluence. By monitoring theelectrical signals from all of the fibers concurrently or sequentially,a spatial distribution of the incident radiation beam in the directionorthogonal to the assembly is obtained. In an alternative embodiment, asmaller number of fibers 404 is used and translated across the radiationbeam. It is possible to use only a single fiber 404 when suchtranslation is used. A two-dimensional beam distribution may be obtainedby using a plurality of the assemblies shown in FIG. 4 oriented at anangle relative to each other. In a particular such embodiment, twoassemblies are disposed at approximately 90 degrees relative to eachother, enabling the two-dimensional distribution of the fluence from theradiation source to be determined.

By enabling the detection of spatial information about the beam withoutsignificant attenuation, the structure of the radiation sensor 168 usingscintillating fibers solves an important practical problem. Conventionalalternatives of using an ionization chamber, for example, suffer from alack of providing spatial details. Alternatives of using image-typedetectors that provide spatial information have the disadvantage ofgreatly attenuating the primary beam.

FIG. 5 illustrates the raw beam profile generated by the radiationsource 112 in the longitudinal direction. Its length in this directiondepends on the opening size of the collimator 116, but a prior-artimaging detector is typically narrower than the beam, as illustrated bythe shaded portion of the drawing. Such prior-art detectors are thusgenerally incapable of directly measuring the beam size. The radiationsensor 168 used in embodiments of the invention can be used for thispurpose during calibration for measuring beam size for smallercollimator openings than maximum. Even when a patient is being scannedand there can be a large degree of scatter outside the primary beam, theradiation sensor 168 of the invention enables determining that thecollimator 116 is correctly positioned.

The fiber structure of the radiation sensor 168 is sufficient to makerelative measurements of the beam profile, but it is generally desirablealso to be able to determine the photon-energy value to enable acalculation of patient dose and air kerma. This may be achieved with asecondary sensor having a gamma-ray response that is different from thatof the scintillating fibers that make up the fiber ribbon. If thesecondary sensor is disposed within the primary beam, its attenuation ofthe beam is preferably low enough not to compromise the ability of thesystem software to correct for that attenuation.

The geometry sensor 172 may take a variety of different forms indifferent embodiments, as may the patient-size sensor 176. Each of thesesensors may use any form of technology that allows for distance or sizemeasurements. Examples include ultrasound technologies in which acoustictransducers reflect acoustic waves from structures comprised by thesystem or from different points on the body of the patient and use theecho time to determine the system geometry or the patient size. Otherexamples include laser micrometers or visual cameras, among a variety ofother distance and size technologies known to those of skill in the art.

Similarly, there are a variety of known technologies that may be used toimplement motion detection. These include a variety of mechanical,electromagnetic, and acoustic technologies that may be used to providethe motion sensor 180. In one embodiment, accelerometers are used formotion detection.

Methods of the invention are summarized with the flow diagram of FIG. 6.While the diagram sets forth a number of functions that may be performedin a particular order, this is not intended to be limiting. Inalternative embodiments of the invention, some additional functions notspecifically identified in the drawing may also be performed, some ofthe functions specifically called out may be omitted, and/or some of thefunctions may be performed in an order different from what is set forth.Part of the method includes determination of a radiation doseadministered to a patient as part of an imaging procedure and may usethe structure described above in connection with FIGS. 1-4.

At block 604, a patient is positioned on the table 108. The patient 104may take a supine or prone position, or may be placed on her side,depending on the type of imaging to be performed, i.e. whether the dataare to be collected to generate a single two-dimensional image, togenerate a series of two-dimensional slice images, or to generate athree-dimensional image. As previously noted, these different types ofimages may be generated using different dynamic configurations of thesystem. Also relevant in the positioning of the patient 104 is whichtissues or structures are to be imaged.

The system geometry is measured with the geometry sensor 172 at block608 and the patient size is measured at block 612 with the patient-sizesensor 176. After the imaging procedure is begun at block 616, anymotion of the table 108 and/or gantry 118 may also be measured at block618, providing a full specification of the dynamical aspects of theprocedure performed on the patient. The fluence is measured at block 620with the radiation sensor 168.

An initial check may be performed on the fluence at block 624 to ensurethat the fluence is within normal parameters. The input arrow to thisblock identifies that information defining standard parameter values maybe obtained and used in the evaluation. A deviation from such normalvalues may prompt the issuance of an alert at block 628 and potentialaborting of the procedure at block 632. Such an alert may take the formof an audible and/or visual alert so that a technician overseeing theprocedure is notified, and the aborting of the procedure at block 632may occur automatically or may result from intervention by such atechnician. The ability to check the fluence at an exit of the radiationsource or collimator permits early intervention, particularly if thefluence is significantly stronger than an acceptable upper limit. Thisallows accidents that might otherwise result from excessive radiation ofa patient to be avoided.

At block 636, a comparison may be made of the measured fluence for theparticular procedure with previous measurements. Such comparisons areuseful in identifying whether there is a systemic decrease in fluencestrength such as may result as the radiation source ages. Such reductionin tube strength is a known consequence of tube aging, requiring the useof higher voltage or current to obtain the desired radiation strength toperform the imaging. If the fluence shows a pattern of decreasing aschecked at block 640, it is possible to calculate an estimated time totube failure at block 644. Such calculations may be performed with avariety of different models of tube behavior that use any number ofparameters, including the current and voltage applied to the radiationsource 112. Such parameters may also include an identification of themanufacturer of the radiation source 112 since the performance-decay oftubes may differ in a predictable way for tubes provided by differentmanufactures. The models used in performing such estimates may also makeuse of past comparisons of fluence levels, which may be collected formultiple systems and recorded by the central system 204 for developingsuch models.

In addition to performing such comparisons during imaging procedures,the presence of the radiation sensor 168 enables measurements also to bemade during calibration procedures. Such calibration procedures aretypically performed at regular intervals for each machine, such as byperforming a daily calibration. It is noted that this determination ofan estimated time to tube failure may be performed without directinformation being supplied by the tube manufacturer, allowing anindependent check on recommendations for tube replacement that may bemade by manufacturers. At block 648, the estimated time to tube failureis accordingly reported, allowing the operator of the machine tointegrate a plan for replacement into its normal operating procedures.

At block 652, the patient dose is calculated. The input arrow to thisblock identifies that parameters used in determining the patient dosemay be obtained and used in the calculation. Such calculations may beperformed in a number of different ways in different embodiments.Typically, some kind of modeling technique is used rather than a directcalculation because of the complexity of accounting for the differentparameters that may impact the actual dose delivered to a patient. Inone embodiment, a Monte Carlo model is used to calculate the dose fromparameters determined from the measurements collected by the sensors.

Such parameters include the effective voltage kV and the peak voltagekVp. The peak voltage is the maximum voltage applied across the x-raytube, defining the kinetic energy of the electrons accelerated withinthe tube and the corresponding peak energy. FIG. 7 provides anillustration of a spectral distribution of a typical radiation beam thatincludes a tungsten anode. The distribution has a characteristic peak atthe tungsten k-edge energy E_(W) and terminates at a maximum energyE_(max), that corresponds to the peak voltage kVp, but otherwise has aspectral distribution at different energies. Other types of radiationsources may be used, such as those that use a molybdenum anode andtherefore have different characteristic peaks. The peak voltage kVp maybe determined by using the secondary sensor having a different gamma-rayresponse as described above. For example, FIGS. 8A and 8B provideillustrations of different responses that two sensors may have, and theratio of the responses allows a calculation of the highest energyphotons in the beam, as known to those of skill in the art.

In addition to kV and kVp, other parameters that may be determined fromsensor measurements and that may accordingly be used in radiation modelsto permit calculation of the dose include the spatial beam intensityprofile, the gantry rotation period, and the patient travel distance,all of which are directly obtained from the radiation sensor 168 and themotion sensor(s). Parameters such as the patient-skin—air kerma with orwithout tube current modulation may be determined from a combination ofpatient girth and imaging geometry as may be obtained from measurementsby the geometry sensor 172 and the patient-size sensor 176. The physicalextent of the dose, i.e. the dose length and the dose volume in helicalscans, may be determined from a combination of geometry measurementsprovided by the geometry sensor 172 and table-motion data as provided bythe motion sensor 180.

Models may use these different parameters in combination with a set oftissue absorption characteristics so that the probability of absorbing aphoton from the beam may be calculated. This probability is dependent onthe photon energy as known from the energy distribution of the beam, onthe spatial distribution of the beam, on the size of the patient, on thespatial interaction size between the patient and the beam, and on thepatient-skin—air kerma, each of which is determined as described above.It is noted that these values may be determined independently fromvalues calculated by the imaging system itself. Provision of suchindependent dose information provides increased safety and enhancedevidence-based quality assurance information. Additionally, decouplingthe calculation of these safety and performance parameters from theimaging system itself greatly increases the probability that systemmalfunctions, especially those that could present a patient-safetyhazard, are detected.

Returning to FIG. 6, a number of different actions may be taken inresponse to calculation of the patient dose. For example, as indicatedat block 656, the patient dose could be output to an operator of theimaging system as part of providing evaluation information for theprocedure. Alternatively or in addition, the patient dose may berecorded, as indicated at block 660, in local and/or central databases,such as central database 208, to provide a record of cumulative dosesthat the patient receives. Such information may aid physicians inevaluating the potential risk of subsequent imaging procedures to beperformed on patients.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. Accordingly, the above description should not be taken aslimiting the scope of the invention, which is defined in the followingclaims.

What is claimed is:
 1. A medical imaging system comprising: a radiationsource having an opening to direct a collimated radiation beam in adirection towards a patient; a radiation sensor disposed proximate theopening and within the collimated radiation beam to measure a fluence ofthe collimated radiation beam; a data-collection unit disposed tocollect radiation from the collimated radiation after interaction withthe patient; an imaging system in communication with the data-collectionunit and configured to generate an image of a portion of the patientfrom the collected radiation; and a monitoring system in communicationwith the radiation sensor, the monitoring system comprising instructionsto determine, during patient imaging, an estimate of an effectiveradiation dose delivered to the patient during an imaging procedure withthe medical imaging system from the measured fluence.
 2. The medicalimaging system recited in claim 1 wherein the radiation sensorcomprises: a scintillating fiber that emits light in response toabsorption of a photon of radiation by the scintillating fiber; and aphotodetector coupled with the scintillating fiber to detect emission oflight by the scintillating fiber.
 3. The medical imaging system recitedin claim 2 wherein: the scintillating fiber comprises a plurality ofscintillating fibers arranged substantially parallel to each other; andthe photodetector comprises a plurality of photodetectors, each of theplurality of photodetectors being coupled with one of the plurality ofscintillating fibers.
 4. The medical imaging system recited in claim 1wherein the radiation sensor comprises: a first radiation sensor having:a first plurality of scintillating fibers arranged substantiallyparallel to each other and to a first direction, each of the firstplurality of scintillating fibers emitting light in response toabsorption of a photon; and a first plurality of photodetectors, each ofthe first plurality of photodetectors coupled with one of the firstplurality of scintillating fibers to detect emission of light by the oneof the first plurality of scintillating fibers; and a second radiationsensor having: a second plurality of scintillating fibers arrangedsubstantially parallel to each other and to a second direction, each ofthe second plurality of scintillating fibers emitting light in responseto absorption of a photon; and a second plurality of photodetectors,each of the second plurality of photodetectors coupled with one of thesecond plurality of scintillating fibers to detect emission of light bythe one of the second plurality of scintillating fibers, wherein thefirst and second directions are nonparallel.
 5. The medical imagingsystem recited in claim 4 wherein the first and second directions aresubstantially orthogonal.
 6. The medical imaging system recited in claim1 wherein the radiation sensor measures a spatial distribution of thefluence.
 7. The medical imaging system recited in claim 1 wherein theradiation sensor measures a spectral distribution of the fluence.
 8. Themedical imaging system recited in claim 1 further comprising a mechanismto effect relative translational and/or rotational motion between theradiation source and the patient.
 9. The medical imaging system recitedin claim 8 wherein the instructions to determine the estimate of theeffective radiation dose delivered to the patient comprise: instructionsto obtain a peak voltage applied to the radiation source to generate thecollimated radiation beam; instructions to obtain a measure of ageometry of the medical imaging system; instructions to obtain a measureof a size of the patient; and instructions to obtain a measure ofrelative motion of the patient with respect to the medical imagingsystem.
 10. The medical imaging system recited in claim 9 furthercomprising a host system in communication with the imaging system andwith the radiation source, wherein: the instructions to obtain the peakvoltage applied to the radiation source comprise instructions to obtainthe peak voltage applied to the radiation source from the host system;the instructions to obtain the measure of the geometry of the medicalimaging system comprise instructions to obtain the measure of thegeometry of the medical imaging system from the host system; theinstructions to obtain the measure of the size of the patient compriseinstructions to obtain the measure of the size of the patient from thehost system; and the instructions to obtain the measure of relativemotion of the patient with respect to the medical imaging systemcomprise instructions to obtain the measure of relative motion of thepatient with respect to the medical imaging system from the host system.11. The medical imaging system recited in claim 9 wherein theinstructions to obtain the peak voltage applied to the radiation sourcecomprise instructions to obtain the peak voltage applied to theradiation source from the measured fluence of the collimated radiationbeam.
 12. The medical imaging system recited in claim 9 furthercomprising a geometry sensor, wherein the instructions to obtain themeasure of the geometry of the medical imaging system compriseinstructions to obtain the measure of the geometry of the medicalimaging system from the geometry sensor.
 13. The medical imaging systemrecited in claim 12 wherein the geometry sensor comprises a sensorselected from the group consisting of an ultrasound sensor, a lasermicrometer, and a visual camera.
 14. The medical imaging system recitedin claim 9 further comprising a patient size sensor, wherein theinstructions to obtain the measure of the size of the patient compriseinstructions to obtain the measure of the size of the patient from thepatient-size sensor.
 15. The medical imaging system recited in claim 14wherein the patient-size sensor comprises a sensor selected from thegroup consisting of an ultrasound sensor, a laser micrometer, and avisual camera.
 16. The medical imaging system recited in claim 9 furthercomprising a motion sensor, wherein the instructions to obtain themeasure of relative motion of the patient with respect to the medicalimaging system comprise instructions to obtain the measure of relativemotion of the patient with respect to the medical imaging system fromthe motion sensor.
 17. The medical imaging system recited in claim 16wherein the motion sensor comprises a mechanical sensor, anelectromagnetic sensor, or an acoustic sensor.
 18. The medical imagingsystem recited in claim 1 wherein the monitoring system is further incommunication with a central system that is in communication with asecond monitoring system remote from the monitoring system.
 19. Themedical imaging system recited in claim 18 wherein the monitoring systemfurther has instructions to record the estimate of the effectiveradiation dose delivered to the patient during the imaging procedure ata data store coupled with the central system.
 20. The medical imagingsystem recited in claim 1 wherein the monitoring system further has:instructions to identify the measured fluence of the collimatedradiation beam as being outside an acceptable range; and instructions toinitiate an alarm in response to identifying the measured fluence beingoutside the acceptable range.
 21. The medical imaging system recited inclaim 1 wherein the monitoring the system further has: instructions toperform a comparison of the measured fluence of the collimated radiationbeam with a record of prior measurements of fluence produced by theradiation source; and instructions to estimate a time to failure of theradiation source from the comparison.
 22. A method of monitoring amedical imaging system comprising a radiation source having an openingto direct a collimated radiation beam in a direction towards a patientand an imaging system configured to generate an image of a portion ofthe patient from radiation collected from the collimated radiation beamafter interaction with the patient, the method comprising: measuring afluence of the collimated radiation beam with a radiation sensordisposed proximate the opening and within the collimated radiation beam;and determining, during patient imaging, an estimate of an effectiveradiation dose delivered to the patient during an imaging procedure withthe medical imaging system from the measured fluence.
 23. The methodrecited in claim 22 wherein the radiation sensor comprises: ascintillating fiber that emits light in response to absorption of aphoton of radiation by the scintillating fiber; and a photodetectorcoupled with the scintillating fiber to detect emission of light by thescintillating fiber.
 24. The method recited in claim 23 wherein: thescintillating fiber comprises a plurality of scintillating fibersarranged substantially parallel to each other; and the photodetectorcomprises a plurality of photodetectors, each of the plurality ofphotodetectors being coupled with one of the plurality of scintillatingfibers.
 25. The method recited in claim 22 wherein the radiation sensorcomprises: a first radiation sensor having: a first plurality ofscintillating fibers arranged substantially parallel to each other andto a first direction, each of the first plurality of scintillatingfibers emitting light in response to absorption of a photon; and a firstplurality of photodetectors, each of the first plurality ofphotodetectors coupled with one of the first plurality of scintillatingfibers to detect emission of light by the one of the first plurality ofscintillating fibers; and a second radiation sensor having: a secondplurality of scintillating fibers arranged substantially parallel toeach other and to a second direction, each of the second plurality ofscintillating fibers emitting light in response to absorption of aphoton; and a second plurality of photodetectors, each of the secondplurality of photodetectors coupled with one of the second plurality ofscintillating fibers to detect emission of light by the one of thesecond plurality of scintillating fibers, wherein the first and seconddirections are nonparallel.
 26. The method recited in claim 25 whereinthe first and second directions are substantially orthogonal.
 27. Themethod recited in claim 22 wherein measuring the fluence of thecollimated radiation beam comprises measuring a spatial distribution ofthe fluence.
 28. The method recited in claim 22 wherein measuring thefluence of the collimated radiation beam comprises measuring a spectraldistribution of the fluence.
 29. The method recited in claim 22 whereinthe medical imaging system further comprises a mechanism to effectrelative translational and/or rotational motion between the radiationsource and the patient, the method further comprising: obtaining a peakvoltage applied to the radiation source to generate the collimatedradiation beam; obtaining a measure of a geometry of the medical imagingsystem; obtaining a measure of a size of the patient; and obtaining ameasure of relative motion of the patient with respect to the medicalimaging system.
 30. The method recited in claim 29 wherein: the medicalimaging system further comprises a host system in communication with theimaging system, the radiation source, and the mechanism; obtaining thepeak voltage applied to the radiation source comprises obtaining thepeak voltage applied to the radiation source from the host system;obtaining the measure of the geometry of the medical imaging systemcomprises obtaining the measure of the geometry of the medical imagingsystem from the host system; obtaining the measure of the size of thepatient comprises obtaining the measure of the size of the patient fromthe host system; and obtaining the measure of relative motion of thepatient with respect to the medical imaging system comprises obtainingthe measure of relative motion of the patient with respect to themedical imaging system from the host system.
 31. The method recited inclaim 29 wherein: obtaining the peak voltage applied to the radiationsource comprises determining the peak voltage applied to the radiationsource from the measured fluence; obtaining the measure of the geometryof the medical imaging system comprises measuring the geometry of themedical imaging system with a geometry sensor; obtaining the measure ofthe size of the patient comprises measuring the size of the patient witha patient-size sensor; and obtaining the measure of relative motion ofthe patient with respect to the medical imaging system comprisesmeasuring the relative motion of the patient with respect to the medicalimaging system with a motion sensor.
 32. The method recited in claim 22further comprising transmitting the estimate of the effective radiationdose delivered to the patient during the imaging procedure to a centralsystem for storing the estimate.
 33. The method recited in claim 22further comprising: identifying the measured fluence of the collimatedradiation beam as being outside an acceptable range; and initiating analarm in response to identifying the measured fluence being outside theacceptable range.
 34. The method recited in claim 22 further comprising:performing a comparison of the measured fluence of the collimatedradiation beam with a record of prior measurements of fluence producedby the radiation source; and estimating a time to failure of theradiation source from the comparison.
 35. The medical imaging systemrecited in claim 1 wherein the medical imaging system is furtherconfigured to store the estimate of the effective radiation dosedelivered to the patient and associate the stored estimate of theeffective radiation dose with the patient.
 36. The medical imagingsystem recited in claim 1 wherein the monitoring system comprisesinstructions to determine the estimate of the effective radiation dosebased on one or more of: a probability of absorbing a photon based on aset of tissue absorption characteristics; and a patient skin-air kerma.37. The method recited in claim 22 further comprising storing theestimate of the effective radiation dose delivered to the patient andassociate the stored estimate of the effective radiation dose with thepatient.
 38. The method recited in claim 22 wherein determining theestimate of the effective radiation dose comprises determining based onone or more of: a probability of absorbing a photon based on a set oftissue absorption characteristics; and a patient skin-air kerma.